If the open_basedir field in the web settings is OK, then it's most likely the
eaccellerator bug (if you have it installed).
I had the same problem some time ago on a very similar setup.
I had to remove eaccellerator - and the problem went away.
On CentOS it should be
yum remove php-eaccelerator
I wish I knew the way to keep it, though, and avoid the bug -so if anyone has any ideas, I'd welcome it very much!